is there a way to mirror traditionally painted textures?
I think you should look at doing your painting in Blender and then 'fixing' it in your external editor. Look at two things to help : The Curves panel, choose a brush curve that gets you a better paint stroke along with the stroke type set to Curve where you can set the points with ctrl-click and then enter key to stroke along the path. 
For Symmetry, your UVs don't need to be lying on top of each other in Blender since you can toggle the symmetry axis there to determine where your paint is mirrored despite your UV island arrangement.

If you must use the external paint then you will need to make your object's UV islands to coincide right and left on top of each other, but I think you might be better off trying the method I laid out here.